Inductive wireless power transfer is becoming increasingly popular. In this technology a power transmitter device generates a magnetic field using a primary coil. A power receiver device taps energy from this magnetic field using a secondary coil, inductively coupled to the primary coil by proximity. Thus power is transferred without making electrical contact. One such technology is standardized in the Wireless Power Consortium, and is known under the name of Qi.
In an application example of this technology, a mobile phone acts as the power receiver and has a secondary coil built in. For charging of the phone's batteries, it is placed on the surface of a wireless charging pad that has a primary coil built in. The two coils are coupled by proper placement and alignment, and power is transferred from the charger to the phone wirelessly by induction. Thus the phone can be charged by simply placing it on a dedicated charger surface, without the need for attaching connectors and wires to the phone. The charging of a mobile phone or other portable device is a low-power application, with typically about 1 to 5 watt of power transferred from transmitter to receiver. High-power applications of inductive wireless power transfer may be used for cooking food or even charging an electrical car wirelessly.
The Qi standard for inductive wireless power transfer specifies a communication interface for communication between a wireless power transmitter and a wireless power receiver. Such communication is needed a.o. to properly match the power transmission to the characteristics of the receiving device. So far, this communication has been specified only for a single transmitter working with a single receiver. Only receiver to transmitter communication is supported, achieved by load modulation on the receiver side. The load modulation leads to modulation of the transmitted power, which can be detected on the transmitter side as modulation of the voltage or current in the primary coil. The single receiver communicates its power needs, and the transmitter obliges. This is described e.g. in WO 2014020464.
A solution is needed for the situation where a single primary coil (or multiple primary coils operated in series or parallel) in a power transmitter is coupled to multiple secondary coils in multiple power receiver devices. This may arise for example in case of a larger charging pad that can accommodate several portable devices simultaneously.